Methods for filtering suspended solids from liquids through filter media frequently rely on filter aids to prefilter the liquid-based mixture and extend the life of the filter media. A filter aid is typically added to a liquid to be filtered prior to processing through a filter medium, or added to the top of a filter medium through which a liquid will be processed. A filter aid inhibits the formation of a densely packed film or cake of solid material that forms on the surface of a filter medium during the process of filtration.
Suspended solids can generally be divided into two groups, deformable and nondeformable. Deformable solids are particularly problematic because they can be compressed readily into densely packed beds, often called flocs or slime. They are primarily biological in origin, and tend to fall into a very specific size range between just under 1 to several dozens of micrometers. Deformable solids are present in higher concentrations in setting water systems (lakes and ponds).
In contrast, moving water systems (rivers, streams, etc.) are likely to have larger concentrations of non-deformable particles. Nondeformable solids are typically inorganic in nature and can vary in dimension from submicrometer to millimeters.
Filter aids function with non-deformable solids by increasing the overall porosity and, therefore, loading capacity of the filtration system. Additionally, filter aids slow the progress of non-deformable solids to the surface of filter media where they can readily penetrate into the pore structure and completely block the flow of liquid. Further, filter aids interrupt the formation of the packed solid film or cake. Filter aids are less efficient in removing non-deformable suspended particles compared to deformable suspended particles.
The Chemical Engineers Handbook (5th Edition, McGraw Hill Book Co., New York (1973) p 19-63) teaches that preferred filter aids are porous and have low bulk density to minimize settling. Conventional filter aids, which are naturally occurring materials, conform to these characteristics. Typical filter aids are diatomaceous earth, sand, cellulose fibers (paper pulp), perlite, fuller's earth, asbestos, sawdust, magnesia, salt, gypsum, and carbon. The most commonly used filter aid is diatomaceous earth.
Additionally, the shape of a material bears on its ability to function as a filter aid. Irregular materials are more susceptible to the formation of very densely packed beds which have correspondingly smaller sized pores which leads to high pressure drop and premature plugging. Another phenomenon associated with the irregular shape is that the surface of these particles can consist of jagged edges which mechanically interlock adjacent particles together and can interfere with bed formation if these locking forces are not overcome. This leads to the formation of "super pores" or channels which negate the intended effect of the filter aid by allowing the suspended solids to pass straight through to the filter medium. The traditional technique to minimize these packing phenomena is to use a low density material that will not rapidly settle out of the liquid and, therefore, will not form a densely packed bed and/or one that has innate porosity which serves to minimize the impact of dense packing.
The use of spherical materials as a filter aid has been broadly described in the prior art. Japanese patent document No. JP59-40767 describes a method for making amorphous non-fused titania particles in the size range of 0.05 to 0.1 micrometer and is broadly disclosed as a filtering agent. U.S. Pat. No. 4,713,338 describes metal oxide based porous spherical beads having a size 5 to 500 micrometers and surface area of 100 to 600 m.sup.2 /g which are stated to be useful for filtration applications, with all utility examples directed to gel permeation chromatography.
U.S. Pat. No. 5,128,291 describes a similar procedure for producing porous titania or zirconia spheres with chromatographic properties and also references the utility of such materials as catalyst supports.
Another standard technique in the filtration industry is to utilize graded beds to maximize particulate removal efficiency. This practice involves forming a series of discrete layers of filter media with the smallest average particle size medium at the bottom of the bed and with succeeding layers having larger particle size than the previous layer. Typical practice utilizes at least two bed layers including anthracite, basalt, tuff, sand, and granular activated carbon with the liquid phase entering from the top of the bed. See for example, A. Adin et al., Filtration & Separation, January/February 1991, pp 33-36.
U.S. Pat. No. 4,153,661 relates to a composite article that can comprise a fibrillated polytetrafluoroethylene (PTFE) matrix with glass beads distributed therein. The article is said to be useful as a semipermeable membrane. U.S. Pat. Nos. 4,810,381, 4,906,378, and 4,971,736 relate to composite chromatographic articles comprising a fibrillated PTFE matrix with non-swellable sorptive particles enmeshed therein and optionally comprising glass beads to act as a property modifier and processing aid.
JP 76032728 (abstract) discloses a method for melt spinning polyamide wherein a two layer filter can comprise a layer of glass beads of 70-150 mesh and a layer of sintered metal or wire mesh.
Beds of large glass spheres have been disclosed as useful models to study filtration processes, as disclosed in C. Ghilaglia et al., Journal of Physics D: Applied Physics, 24 (1991) 2111-2114.